Under the Clean Air Act of 1990, the Environmental Protection Agency (EPA) was given the authority to set the maximum levels of certain air pollutants anywhere in the United States. Since most of these pollutants were derived from automotive exhausts, the concept of reformulated gasoline (RFG) was introduced by the EPA in order to help the cities and states with the highest levels of pollution meet the minimum requirements of the National Ambient Air Quality Standards, especially with respect to ozone concentration. Both methyl tertiary butyl ether (MTBE) and ethanol were approved as oxygenated gasoline additives for this purpose. Although domestic refiners have used MTBE for over a decade, recent studies have found MTBE to be carcinogenic and a source of groundwater contamination from leaking gasoline storage tanks. California and fifteen other states have subsequently instituted MTBE bans.
The inherent problems of MTBE as a fuel additive prompted renewed interest in ethanol for this purpose. Accordingly, Congress passed the Energy Policy Act of 2005, creating for the first time a Renewable Fuels Standard (RFS) that committed the United States to the use of ethanol as a replacement for MTBE in gasoline and established a baseline ethanol usage of 4 billion gallons in 2006. Approximately 30% of the gasoline now sold in the United States contains ethanol.
Congress subsequently passed the Energy Independence and Security Act of 2007, which sets a mandatory RFS requiring fuel producers to use at least 36 billion gallons of biofuels per year by 2022. The term biofuels includes ethanol, butanol and biodiesel but primarily refers to ethanol. Ethanol is primarily derived agriculturally from corn. In processes used today, ethanol is produced primarily from corn or sugar by fermentation of sugars or starches in either a batch or continuous process. The mash is heated prior to fermentation to eliminate harmful bacteria that would otherwise impede the fermentation process. After heating, the mash is transferred to a fermentation tank, and yeast is added to promote the production of ethanol, which typically takes 40-50 hours. The tank is agitated during fermentation by either a mechanical stirrer or by a gaseous air lift. The fermentation product is a dilute aqueous ethanol stream that is commonly called “beer”, which contains up to about 16-18 percent ethanol by volume.
The use of corn as a fuel source competes with the use of corn as a food product, and present production capacity does not allow the United States to grow enough corn to meet demand for both. Accordingly, production of alcohols, including ethanol and butanol, from non-food (i.e., cellulosic) feedstocks will be required in order to meet the RFS in 2022. Future production of ethanol and butanol will require production from cellulosic feeds such as, for example, switch grass, corn stover, bagasse, tree bark and sawdust. Several routes have been proposed for the production of ethanol from cellulosic feedstocks. The first route involves enzymatic breakdown of cellulose and hemicellulose structures to form starches that can be subsequently fermented. The second route involves a thermochemical method in which the cellulosic feedstock is first gasified to produce synthesis gas (CO and H2), which is then converted to ethanol or a mixture of alcohols and liquid hydrocarbons, all of which are usable as fuels. The enzymatic processes and at least some of the thermochemical processes produce “beer” in a dilute aqueous ethanol stream having only about 3-5 percent ethanol by volume.
In order to recover ethanol from dilute aqueous ethanol streams, the liquid (either with or without filtration to remove solids) is typically fed to a multi-stage distillation apparatus, which produces a primary overhead product containing about 93-95 weight percent ethanol. Higher ethanol content cannot be achieved by distillation, since ethanol and water form a binary azeotrope at 95.6 weight percent ethanol. An additional processing step is conventionally used for further water removal such as, for example, adsorption of water by zeolite molecular sieves to form a fuel-grade ethanol stream containing greater than about 99 weight percent ethanol. Both the distillation and the drying steps are extremely energy intensive processes, the energy input of which is comparable to that of the energy output achieved when the ethanol is burned as a fuel.
In view of the foregoing, there remains a need for efficient and economical methods for separation of ethanol and other alcohols from dilute aqueous alcohol feed streams, particularly those that do not require distillation to affect separation.